Fluorescent nanotubes used for imaging of internal organs

An enhanced color image of fluorescence from single-walled carbon nanotubes (right) shows internal organs of a mouse next to a reference illustration (left) (Image: PNAS)

Mice are frequently used as lab models when testing new drugs, and fluorescent dyes are sometimes injected into their bodies so that researchers can better see how those drugs are progressing through their systems. Unfortunately, the pictures obtained in this process start to become murky when imaging anything more than a few millimeters beneath the skin. Scientists from Stanford University have now devised a system that utilizes fluorescent carbon nanotubes to produce clear color images of organs that are located centimeters within a mouse's body.

Attached to a drug dosage, the single-walled nanotubes fluoresce brightly when exposed to a laser beam directed at the mouse. A video camera tuned in to the near-infrared wavelengths emitted by the nanotubes records the images.

Dyes fluoresce at wavelengths below 900 nanometers, which is the same range at which biological tissues naturally fluoresce. As a result, in images obtained using the dye method, it is often difficult to distinguish between the mouse and the dye. Because the nanotubes fluoresce between 1,000 and 1,400 nanometers, however, the background noise in images obtained using them is minimal. The images are also sharper due to the fact that biological tissue scatters light less at the longer wavelengths used by the nanotubes.

Additionally, it is possible to fine-tune the wavelength at which the nanotubes fluoresce. Running the video footage through a process known as "principal component analysis," it is then possible to distinguish between subtly-different signals, resulting in images in which different organs appear in different colors.

Techniques such as magnetic resonance imaging (MRI) and computer tomography (CT) scanning are able to obtain images from deeper within the body than the nanotubes, but require advanced machinery that isn't always available for mouse-based drug trials.